Each mobile radio communication terminal device such as a user equipment (UE) in a cellular system needs to be aware of the cellular timing (in other words of the timing of the transmission of radio signals e.g. from a base station, e.g. a NodeB or eNodeB), which is normally derived from the timing of a received radio signal. Therefore, a timer is usually provided in the UE system maintaining the cell-time. This is especially true in time periods in which no radio signal reception takes place in a UE such as a time period in which the UE is in Radio Resource Control (RRC) IDLE mode, e.g. in RRC Discontinuous Reception (DRX), etc. In a situation in which the radio signal reception is ongoing, the timer frequency and the sampling frequency should be aligned. Any drift between the timer and the sampling frequency may cause call-drops.
In a conventional cellular system, there is usually a fixed coupling between timer and sampling frequency and the frequency corrections are applied to a reference frequency, which simultaneously corrects the timer and the sampling.
This is different in cellular system with a so-called free-running reference, which is used for sharing frequency between several wireless systems or even within one single cellular modem in one UE between different Subscriber Identity Modules (SIMs). In a free-running system, the timer and the sampling might be corrected separately, leading to critical error accumulation in the long run.
Another problem may arise if the sampling frequency is derived from the radio frequency (RF) directly and varying heavily with the carrier frequency. In this case it would cost high hardware (HW) effort to keep sampling and timer frequencies identical.